1. Field of the Invention
The present invention relates to a step-up switching voltage regulator circuit mounted on a portable device.
2. Description of the Related Art
Portable devices such as a cellular phone and a personal digital assistance (PDA) are driven by a power voltage supplied from a battery (including a secondary battery) mounted on the portable devices. The power voltage supplied from the battery drops with use of the portable device and the like. Therefore, a voltage step-up circuit for raising the dropped power voltage to a predetermined output voltage is mounted on the above-described portable device so that the portable device can be driven, even when the power voltage supplied from the battery drops. As one of such voltage step-up circuits, there is a step-up switching voltage regulator circuit.
FIG. 9 is a diagram showing a conventional step-up up switching voltage regulator circuit shown in FIG. 9 is a voltage step-up chopper circuit. The step-up switching voltage regulator circuit is provided with a reference voltage generation circuit 1, an error amplifier 3, a PWM (Pulse-Width Modulation) circuit 5, a driver circuit 7, a switching transistor 9a, a diode 9b, a coil 11, and a capacitor 13. Furthermore, this step-up switching voltage regulator circuit raises a power voltage Vi supplied from a battery to a predetermined output voltage Vo to output the voltage.
The reference voltage generation circuit 1 is a circuit which generates a reference voltage Vb for setting the predetermined output voltage Vo of the step-up switching voltage regulator circuit. An output section of the reference voltage generation circuit 1 is connected to one of input sections of the error amplifier 3. The error amplifier 3 is a circuit for comparing the reference voltage Vb with the predetermined output voltage Vo output from the step-up switching voltage regulator circuit to output an error signal based on a result of the comparison. An output section of the error amplifier 3 is connected to an input section of the PWM circuit 5. It is to be noted that the error signal increases with an increase of a difference between the reference voltage Vb and the output voltage Vo.
The PWM circuit 5 sets a duty ratio of a PWM signal based on the error signal output from the error amplifier 3 to output the duty ratio. The duty ratio of the PWM signal increases with an increase of a value of the error signal. Furthermore, an output section of the PWM circuit 5 is connected to an input section of the driver circuit 7. It is to be noted that if the PWM circuit 5 compares a triangular wave with the error signal, the PWM signal becomes a low level when the triangular wave is greater than the error signal.
The driver circuit 7 is an amplifying circuit which amplifies an amplitude of the PWM signal output from the PWM circuit 5 up to such an extent as to enable switching control of the switching transistor 9a described later. An output section of this driver circuit 7 is connected to a gate of the switching transistor 9a. 
The switching transistor 9a is an n-channel type FET (Field-Effect Transistor) which turns on when the PWM signal output from the driver circuit 7 is applied to a gate and this PWM signal is at a high level. A drain of the switching transistor 9a is connected to the coil 11, and a source thereof is grounded. A quantity of a current flowing through the coil 11 is controlled in accordance with the switching control of the switching transistor 9a. One terminal of the coil 11 is connected to the battery, and the other terminal thereof is connected to an anode of the diode 9b and the drain of the switching transistor 9a. 
The diode 9b prevents backflow of the current, when the capacitor 13 described later discharges. The anode of this diode 9b is connected to the coil 11, and a cathode thereof is connected to one terminal of the capacitor 13. The capacitor 13 holds a voltage of the diode 9b on a cathode side. One terminal of the capacitor 13 is grounded, and the other terminal thereof is connected to an output terminal of the step-up switching voltage regulator circuit, whereby the output voltage Vo is output from this output terminal. This output voltage Vo is fed back to one of input terminals of the above error amplifier 3.
According to the above-described constitution, the conventional step-up switching voltage regulator circuit shown in FIG. 9 raises the power voltage Vi supplied from the battery to the predetermined output voltage Vo, and then outputs the raised voltage. Next, an operation of the conventional step-up switching voltage regulator circuit will be described.
The reference voltage Vb generated in the reference voltage generation circuit 1 is input into one input terminal of the error amplifier 3. On the other hand, the output voltage Vo is input into the other input terminal of the error amplifier 3.
Here, the output voltage Vo is a voltage (Vo=Vi−Vf) obtained by subtracting a forward-direction voltage Vf of the diode 9b from the power voltage Vi. The error amplifier 3 compares two input voltage values, and outputs the error signal in accordance with the difference between the output voltage Vo and the reference voltage Vb. It is to be noted that the error signal increases with the increase of the difference between the reference voltage Vb and the output voltage Vo, as described above.
Next, the PWM circuit 5 sets the duty ratio of the output PWM signal in accordance with the error signal output from the error amplifier 3, and outputs the PWM signal whose duty ratio has been set. Here, as described above, the duty ratio of the PWM signal is output at a large value (a time of the high level lengthens), as the value of the error signal is large. It is to be noted that if the PWM circuit 5 compares the triangular wave with the error signal and when the triangular wave is greater than the error signal, the PWM signal becomes the low level.
The PWM signal output from the PWM circuit 5 is input into the driver circuit 7, and is amplified to such an extent that the switching control of the switching transistor 9a is possible. The PWM signal amplified by the driver circuit 7 is applied to the gate of the switching transistor 9a to subject the switching transistor 9a to switching control. This switching control will be described hereinafter in detail.
When the PWM signal output from the PWM circuit 5 and amplified by the driver circuit 7 is at the high level, the switching transistor 9a turns on. At this time, the current flows through the coil 11. Next, when the PWM signal output from the PWM circuit 5 and amplified by the driver circuit 7 becomes the low level, the switching transistor 9a turns off. At this time, a coil voltage V1 is generated in the coil 11 in accordance with a change of the quantity of the flowing current. Accordingly, the output voltage Vo becomes a voltage (Vo=Vi+V1) obtained by adding the power voltage Vi to the coil voltage V1 generated in the coil 11. With this voltage Vo, the capacitor 13 is charged via the diode 9b. In this manner, the power voltage Vi is raised by the switching control of the switching transistor 9a, and the raised voltage Vo is output as the output voltage.
As understood from the above, the step-up switching voltage regulator circuit raises the power voltage, as the power voltage supplied from the battery drops. Therefore, the step-up switching voltage regulator circuit can output the predetermined output voltage, even if the power voltage supplied from the battery drops.
On the other hand, the power voltage of the battery mounted on the above-described portable device exceeds a voltage necessary to drive the portable device sometimes. For example, in a 3V-specified Li (Lithium) ion battery mounted on the cellular phone or the like, a power voltage of 4 V or more is obtained when the battery is fully charged. However, the output voltage necessary to operate the portable device is 4 V or less (e.g., a voltage of about +3.3 V is sufficient for driving an IC (Integrated Circuit)).
Therefore, when the step-up switching voltage regulator circuit is driven in a case where the power voltage supplied from the battery is such a voltage as to enable the driving of the portable device, the power is consumed in vain, and exhaustion of the battery is accelerated. Even if the power voltage Vi is output without the rise of the voltage, a burden on the battery mounted on the portable device increases as much as a drop of the forward-direction voltage Vf of the diode. FIG. 10 is a diagram showing a behavior of a consumed current i of the battery which increases owing to the drop of the power voltage Vi. As shown in FIG. 10, the predetermined output voltage Vo unnecessarily rises as much as the drop due to the forward-direction voltage Vf of the diode 9b, so that the current i of the battery is inconveniently consumed, and the burden on the battery increases.
An object of the present invention is to realize a step-up switching voltage regulator circuit which reduces a burden on a battery mounted on a portable device.